Vinyl chloride based resins are inexpensive as well as having characteristics such as superior processability, chemical resistance, durability, dielectric property, and the like. Due to such characteristics, vinyl chloride based resins are used as hard and soft materials in pipes, wall papers, window frames, wire covers, wrap films, sheets, and the like. However, since vinyl chloride based resins are broadly utilized and customer demand for processability has been rising, continuous improvement in processability is required.
Generally, the processability of a vinyl chloride based resin is affected by factors such as porosity of particles, particle diameter, particle distribution and morphology. The meltability of resins has been improved by controlling the factors. The meltability of resins is estimated by a protrusion or fish-eyes, which is a quality standard to estimate the meltability. Quality of a final product is estimated by generation of a protrusion or fish-eyes.
Quality degradation of the protrusion is caused by variation in metability among particles. In addition, generation of unmelted particles is related to the foam generated during reaction. In detail, foam is generated when a reflux condenser is often utilized in the late of the reaction and at the same time, pressure drops. At this time, dry foam including particles floats in an upper portion of a reactor and in a reflux condenser. However, since it is difficult to sufficiently wash all batches, particles floated by the foam remain inside the reactor and reflux condenser. The remaining particles are introduced into subsequent reactions and thereby, repolymerized. As a result, large and low porosity particles are generated. Due to such particles having low meltability, quality of the protrusion deteriorates.
Prior art to solve the problem is described in U.S. Pat. No. 5,087,678 and Japanese Application Pub. No. 2007-284510. In the patent literature, a method using an antifoaming agent alone or in combination with other additives has been used to inhibit foaming during the reaction. However, by the method using the antifoaming agent alone, it is difficult to inhibit generation of the dry foam. Furthermore, by adding an antifoaming agent and additives into the reactor during the reaction, expenses for equipment, an antifoaming agent and additives are increased. Additionally, resin quality may deteriorate due to addition of the antifoaming agent